The invention relates to Radio Frequency Identification (RFID) systems and, particularly, to an RFID system designed to continuously track articles and personnel as they move through buildings.
RFID products typically have three components: (1) a tag (the item being identified), (2) an interrogator (a device which detects the presence of a tag), and (3) a system (typically including cabling, computers, and software which tie together the tags and interrogators into a useful solution). RFID products are typically designed to detect tags when they pass within range of a few fixed or handheld interrogators.
RFID systems are usually deployed as high-end replacement technology for bar coding. RFID and related systems include passive RFID systems, active RFID systems, infrared ID systems, and Electronic Article Surveillance (EAS) systems.
The tags in a passive RFID system do not carry on-board power. The interrogator in such systems transmits operating power for the tags. Such systems generally have a detection range of a meter or less, although somewhat longer ranges have been achieved. Typically, these systems operate in the 125-kilohertz radio band.
Most passive RFID systems work as follows. An interrogator emits an electromagnetic field for the purpose of powering the tag. A coil in the tag is powered by the electromagnetic field, causing the tag's circuitry to "wake up." The tag uses this power to send an identifying signal back to the interrogator.
Although most passive RFID systems are read-only (that is, the tags in such system respond to a query by reading information from their memory and sending the information back to the interrogator), the tags used in some passive RFID systems have a limited ability to accept information and instructions from the interrogator, for example read/write capabilities in smart cards (electronic money) and "electronic manifests" in industrial applications.
Passive RFID tags have been employed in conjunction with access control, smart cards, vehicle identification (AVI), waste management, item tracking, animal identification, manufacturing control, materials handling, and a variety of other purposes.
One fundamental design goal of any RFID system is for the weak signal emitted from the tag to be distinguishable from the much stronger signal emitted by the interrogator. Some strategies for doing this include:
Frequency shifting. Circuitry in the tag receives a carrier from the interrogator, translates the signal to another frequency, and emits a response modulated onto that second frequency.
Half duplex operation. The tag is charged by the interrogator. When the interrogator's charging circuit turns off, the tag uses the stored power to respond.
Modulated backscatter. The tag modulates its antenna cross section to identify itself to the interrogator.
Delayed retransmission. Surface acoustic wave (SAW) devices retransmit the interrogator's carrier after a delay. The tag's identity is indicated by time variations in the delayed response.
Active RFID systems require battery-powered tags. The battery permits a longer detection range of between 3 and 100 meters. These systems are capable of locating tags with higher accuracy than passive RFID systems and typically operate in the 400, 900, or 2440 megahertz bands. Active tags tend to enable multiple tags to be within range of an interrogator by the use of "handshaking" between the tags and interrogator, so that each tag transmits its signal in turn. Communication between tag and interrogator in active RFID systems is also typically faster than with passive tags.
Most active RFID tags respond to the interrogator when polled, in accordance with a communications protocol. Some active RFID tags "chirp" (transmit) a signal spontaneously at predetermined intervals. A tag's chirped signal is detected by the interrogator if the tag is in range of the interrogator.
Infrared systems (IRID), while not RFID systems, also endeavor to detect and identify the position of mobile tags. A typical IRID system includes a tag that chirps its identity at randomized intervals. Infrared readers located in the ceiling detect these transmissions, and report them to a host. The transmission rate from the tag to reader is typically about 600 baud. Motion detectors in the tags enable the tags to transmit more frequently when in motion. The tags are typically about the size of dominos.
EAS systems are intended to deter theft in retail environments. EAS tags are fairly unreliable, very low in cost, and limited in capabilities. Although they track mobile tags, they are not generally considered to be RFID products, because EAS tags are uncoded and cannot be distinguished from one another.